For most people, violent media doesn't do that much other than likely desensitizing people to violent behaviour a little bit at a time. This doesn't cause people to go on a killing spree because most people are capable of reasoning out their actions.
I work with young adults with Fetal Alcohol Spectrum Disorder and many of the guys I work with often don't have an ability to reason out their actions. If something makes them upset (I took too long to go to the washroom, their social assistance cheque didn't come in today, the sun is yellow) they can get extremely upset.
I have seen first hand how much more aggressive and violent they get after putting on a 50 Cent CD and playing shooting games. Often times, it results in one of my guys ending up in jail again.
Do I blame 50 Cent, Halo or GTA? No, but I can appreciate that under certain conditions, they can be the factor that pushes someone over the edge, or brings them to a heightened level of emotion where the next "problem" will push them over the edge.
This doesn't account for everyone, but I wouldn't be surprised if there are a great deal of "normal people" that could be pushed over the edge with violent material in the same way.